The invention described herein arose in the course of, or under, contract No. W-7405-ENG-48 between the United States Department of Energy and the University of California for the operation of Lawrence Livermore National Laboratory.
The present invention relates generally to laser technology and more particularly to a means and method for the correction of laser beam wavefront distortion.
In the processing and projection of laser beams it frequently occurs that an unwanted side effect of such processing is distortion of the wavefront of the laser beam. As one skilled in the art will recognized, this is a generally undesirable condition, since any wavefront form other than perfectly flat will result in distortion of the intensity distribution of the laser beam in the far field. One form of such distortion is astigmatism, and it is known in the art to correct such distortion by causing the laser beam to be projected onto a mirror, which mirror is curved as required to correct the astigmatism. U.S. Pat. No. 4,647,164 issued to the present inventor and William Sweatt has taught a means for variably contouring such a correction mirror so that correction can be made for curvature of higher orders. U.S. Pat. No. 4,664,488, also issued to the present inventor and Sweatt, has taught an improved means for accomplishing the correction of astigmatism in laser beams, which improved means has a bendable mirrored surface with a plurality of legs projecting perpendicularly from the back thereof. As is disclosed in the '488 Patent, the mirrored surface may be controllably distorted by selectively applying forces to the projecting legs, and the means for doing this is described in detail in that Patent.
Despite the improvements in the field relating to the correction of astigmatism in laser beams, the present inventor and others engaged in related pursuits have been concerned that there are other forms of distortion which are commonly introduced onto laser beam wavefronts which are not so easily correctable by any known prior art means. Among these more difficult problems is an optical phenomenon known as COMMon Aberration in laser beams ("COMA"). COMA can be corrected by aspherically manufactured optical elements, but not for an arbitrary magnitude of distortion, which is required in some laser systems. Further, since COMA imparts a form to the laser wavefront having a component which is described by a cubic function, it cannot be corrected by a conventional cylindrically curved mirror.
The invention of the above referenced '488 Patent, in addition to offering the substantial improvement of better controllability in bending mirrors to shapes defined by quadratic functions, attempted to address the problem of bending the mirror into shapes defined by cubic functions. Since it was not thought to be possible to bend a mirror into a shape defined by a cubic function by applying bending forces on the mirror relative only to itself, the '488 Patent disclosed an additional apparatus for applying bending forces on the mirror relative to the apparatus on which the mirror was mounted. This was, at the time, the best known means for attempting to bend a mirror into shapes defined by cubic functions, and it did indeed approximate that result. However, although the rearwardly projecting legs of the '488 Patent have proven to be quite useful, the additional means for applying forces relative to the mirror mounting has been far less than entirely successful. In practice, the additional means for applying forces relative to the mirror mounting has encountered some considerable difficulties, including that it is difficult to precisely control forces on the mirror with it and further in that, in addition to the desired forces, the external mechanism necessarily applies an unwanted force which tends to turn the mirror out of alignment relative to the mounting.
Clearly, it would be advantageous to have a convenient means for producing a mirror which can correct for COMA. Further, it would clearly be advantageous to have a means for producing a COMA correction mirror which is adjustable to suit varying application requirements, much as in the manner of the adjustable mirror taught by the '488 Patent, although without the necessity of applying forces on the mirror relative to its mounting, as was taught by the '488 Patent. However, despite the considerable related developments which have occurred in this field, those skilled in the art have believed that it would not be practical to attempt to deform a flat mirror to a form required to correct for COMA using internal forces only.
No prior art mirror, to the inventor's knowledge, has successfully provided a means for correcting for COMA with the degree of efficiency and precision attainable by the present invention. All prior art means have either required costly mirrors which require complex control systems and/or have provided a less than desirable accuracy of correction. Furthermore, no prior art mirror, to the inventor's knowledge, has provided an adjustable means for correcting for COMA which did not also result in unwanted forces being applied to the mirror relative to its mounting.